Chap. XVI. CONSPICUOUS COLOURS. 229 



the first year, and of tlie adults during the winter, are 

 either pure white, or much paler-coloured than during 

 the breedinof-season. These latter cases offer another 

 instance of the capricious manner in which sexual se- 

 lection appears often to have acted.^* 



The cause of aquatic birds having acquired a white 

 plumage so much more frequently than terrestrial birds, 

 probably depends on their large size and strong powers 

 of flight, so that they can easily defend themselves or 

 escape from birds of prey, to which moreover they are 

 not much exposed. Consequently sexual selection has 

 not here been interfered with or guided for the sake of 

 protection. No doubt, with birds which roam over the 

 open ocean, the males and females could find each 

 other much more easily when made conspicuous either 

 by being perfectly Avhite, or intensely black ; so that 

 these colours may possibly serve the same end as the 

 call-notes of many land-birds. A white or black bird, 

 when it discovers and flies down to a carcase floating 

 on the sea or cast up on the beach, will be seen from 

 a great distance, and will guide other birds of the same 

 and of distinct species, to the prey ; but as this would 

 be a disadvantasre to the first finders, the individuals 

 which were the whitest or blackest would not thus have 

 procured more food than the less strougly coloured 

 individuals. Hence conspicuous colours cannot have 

 been gradually acquired for this purpose through na- 

 tural selection.^^ 



^■* Oil Lams, Gavia, and Sterna, see Macgillivray, ' Hist, Brit. Birds, ' 

 vol. V. p. 515, 584, 62G. On the Anser liyperboreus, Audubon, ' Oruitli. 

 Biography,' vol. iv. p. 562. On the Anastomus, Mr. Blyth, in ' Ibis,' 

 1867, p. 173. 



■" It may be noticed that with vultures, which roam far and wide 

 through the higher regions of the atmosphere, like marine birds over 

 the ocean, three or four species are almost wholly or largely Avhite, and 



